[Pw_forum] elphonon.f90, question about q=0 case, wavefunction evc not used

Fan Zheng zhengfan at sas.upenn.edu
Sun Nov 22 23:59:05 CET 2015

Dear all,

I have a question about the electron phonon coupling calculation in the
subroutine "elphel" (in the file elphonon.f90).

This subroutine computes <n,k|dV|m,k+q>. |n,k> is the wavefunction for band
"m" with wavevector "k".

First, the program computes <n,k|dV.  Then, it is multiplied by |m,k+q>

If q=0, it only needs to read to evc as:
        IF (lgamma) THEN
           CALL get_buffer(evc, lrwfc, iuwfc, ikk)
           CALL get_buffer (evc, lrwfc, iuwfc, ikk)
           CALL get_buffer (evq, lrwfc, iuwfc, ikq)
My question is in the q=0 case, array "evq" is used to multiply the
computed <n,k|dV instead of evc. (line 401-406 in version 5.2.1)

Can anyone explain  why "evc" array is not used in q=0 case for these line

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